This invention relates to the field of Integrated Circuit (IC) fabrication and assembly, and in particular to the fabrication of light emitting device (LED).
As the light emitting capabilities of semiconductor light emitting devices (LEDs) continues to improve, their use in conventional lighting applications continues to increase, as does the competitive pressures to provide reliable, long-lasting products in a cost-effective manner. Even though the cost of LED products is relatively low, the savings of even a few cents per device can have a significant impact on profit margin, due to the increasingly growing market for these devices.
Advances have continued to be made in reducing the size of the individual die that provides the light emitting device, thereby reducing the material costs, but handling considerations require that the die be mounted on a larger substrate. That is, for example, although semiconductor manufacturers have equipment that can pick and place individual dies that may be as small as 1 mm×1 mm, and provide connections to contacts on each die, the equipment that is conventionally used for printed circuit manufacturing is not suitable for placing or connecting such dies. In like manner, light emitting devices are often plug-in replaceable items, and need to be large enough to be gripped by human hands.
FIGS. 1A-1B illustrate an example substrate 150 for mounting a LED die 110. The substrate 150 is commonly a ceramic, such as aluminum nitrate (AlN) or Alumina (Al2O3), and the combination of a die 110 and substrate 150 is typically referred to as a Device on Ceramic (DoC).
The example LED die 110 of FIG. 1 illustrates the use of four light emitting elements 116. Contacts 112 enable the coupling of the light emitting elements to an external source of power. In this example, pressure-clips 122 hold the die 110 to the substrate 150, as well as providing electrical coupling to the contacts 112. The use of pressure clips facilitates the replacement of the die 110.
Pads 120 provide for attaching the substrate mounted LED device to an external source of power, and may be used, for example, as bond pads for bonding the device to a printed circuit board or similar structure. The bond pads 120 provide the same function as the contacts 112, but, as noted above, are significantly larger, to facilitate common printed circuit board manufacturing equipment and techniques.
The substrate 150 also serves as a heat sink for dissipating the heat generated by the light emitting elements 116.
FIG. 1C illustrates an alternative arrangement, wherein the LED die 110 is soldered to the pads 120 on the substrate 150, via solder elements 122. In this example embodiment, one or more vias 115, or other internal routing in the die 110, provide the coupling to the light emitting elements 116. In the example of FIG. 1C, the upper contacts 112 are also provided, so that the LED die can be used in either of the configurations of FIG. 1B or 1C.
The example of FIG. 1C also illustrates the use of a heat sink pad 130 to which the LED die is soldered. This arrangement will typically provide for an improved thermal coupling between the LED die 110 and the substrate 150, but generally requires a corresponding solderable heat transfer contact 118 on the lower surface of the die 110.
Although the placing of conductive traces on ceramic, such as the pads 120, 130, is a mature technology, there is a cost involved with the fabrication of ceramic substrates with such pads 120, 130, as well as some risk of separation of the pads 120, 130 from the substrate due to the mismatch between the thermal coefficient of the metals commonly used to provide these pads and the thermal coefficient of the ceramic substrate.